Variable optical filter and a wavelength-selective sensor based thereon

ABSTRACT

A variable optical filter is disclosed including a bandpass filter and a blocking filter. The bandpass filter includes a stack of alternating first and second layers, and the blocking filter includes a stack of alternating third and fourth layers. The first, second and fourth materials each comprise different materials, so that a refractive index of the first material is smaller than a refractive index of the second material, which is smaller than a refractive index of the fourth material; while an absorption coefficient of the second material is smaller than an absorption coefficient of the fourth material. The materials can be selected to ensure high index contrast in the blocking filter and low optical losses in the bandpass filter. The first to fourth layers can be deposited directly on a photodetector array.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/837,180 filed Dec. 11, 2017 (now U.S. Pat. No. 10,312,277), which isa continuation of U.S. patent application Ser. No. 15/337,489, filedOct. 28, 2016 (now U.S. Pat. No. 9,842,873), which is a continuation ofU.S. patent application Ser. No. 15/017,583, filed Feb. 5, 2016 (nowU.S. Pat. No. 9,515,119), which is a divisional of U.S. patentapplication Ser. No. 14/166,747, filed Jan. 28, 2014 (now U.S. Pat. No.9,261,634), which claims priority from U.S. Provisional PatentApplication No. 61/757,846, filed Jan. 29, 2013, the disclosures ofwhich are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to optical filters, and in particular tooptical filters having spatially varying spectral characteristics.

BACKGROUND OF THE INVENTION

A spatially variable optical filter has a transmission wavelengthvarying in a transverse direction across the filter. A compact opticalspectrometer can be constructed by attaching a photodetector array to aspatially variable optical filter. A filter having the transmissionwavelength varying linearly with distance in a transverse directionacross the filter is called a linearly variable filter (LVF). Linearvariation of the transmission wavelength with distance is convenient,although not necessary. Optical spectra obtained using an LVF and aconstant-pitch photodetector array have a constant wavelength step.

Pellicori et al. in U.S. Pat. No. 4,957,371 disclose a wedge-filterspectrometer including a LVF having a first plurality of layers of highindex of refraction material and a second plurality of layers of lowindex of refraction material, individual high- and low-index layersoverlapping each other and having a substantially linearly taperedthickness, to form a linearly variable optical thin film interferencefilter. A photodetector array is attached to the LVF, resulting in avery compact overall construction.

Anthon in U.S. Pat. No. 6,057,925 discloses a compact spectrometerdevice including a thin film interference LVF and a photodetector arraycoupled to the LVF via an array of gradient-index lenses or an array ofmicrolenses, for use in a color sensing device, such as a portablecolorimeter. Lightweight and robust construction of the thin filminterference LVF-based spectrometer allows the portable colorimeter tocharacterize color of articles in field conditions.

Weigl et al. in U.S. Pat. No. 6,091,502 disclose a compact LVF-basedspectrometer for performing fluorescence and absorption spectralmeasurements in flow cells with spatial resolution. By placing the LVFin an optical path, such that the transmission variation of the filteroccurs in the flow direction, it is possible to spectroscopicallydetermine concentration of dye markers of proteins in a flow ofbiological cells.

Referring to FIG. 1A, a typical prior-art compact optical spectrometer100, similar to those used in Pellicori, Anthon, and Weigl devices,includes a LVF 102 optically coupled to a photodetector array 104.Transmission wavelength λ_(T) varies in a direction 106 across the LVF102. In operation, light 108 impinges onto the LVF 102. The LVF 102passes through only a narrow wavelength band around the transmissionwavelength λ_(T), which varies in the direction 106 parallel to thephotodetector array 104. As a result, each photodetector 105 of thephotodetector array 104 is responsive to a different wavelength band ofthe light 108. By measuring photocurrents of each photodetector 105 ofthe photodetector array 104, an optical spectrum of the light 108 can beobtained.

The LVF 102 includes a thin film stack 112 supported by a substrate 110.Referring to FIG. 1B, the thin film stack 112 includes two regions: ablock region 121 for blocking wavelengths shorter than and longer thanλ_(T), and a bandpass region 122 for transmitting only a narrow passbandcentered around λ_(T). Each of the two regions 121 and 122 includesalternating high-index layers 131 and low-index layers 132 having highand low refractive indices, respectively. The materials of thehigh-index 131/low-index 132 layers are the same across the regions 121and 122, only the thicknesses are varied to achieve the opticalperformance required. The blocking region 121 includes quarter-wavestacks for blocking wavelengths other than λ_(T), and the bandpassregion 122 half-wave stacks for transmitting the narrow passbandcentered around λ_(T). The material combinations in the material paircan include metal oxides or fluorides.

One drawback of the LVF 102 is an inherent tradeoff between opticalperformance of the LVF 102 and the overall thickness of the thin filmstack 112. To ensure good blocking of the wavelengths other than λ_(T),the blocking region 121 has to include many layers. For low-loss oxides,the number of layers can be up to a hundred layers. To ensure narrowpassband around λ_(T), the bandpass region 122 also needs to includemany layers, and/or to include a thick central layer. Large thickness ofthe thin film stack 112 results in an increase of internal stresses inthe thin film stack 112, causing it to break and/or delaminate from thesubstrate 110. High-index material, such as silicon, can be used toreduce the overall number of layers. However, high-index materialstypically increase optical loss of the LVF 102.

SUMMARY OF THE INVENTION

It is a goal of the invention to alleviate the tradeoff betweenthickness and optical performance of a variable optical filter.

The inventors have realized that key optical requirements for materialsin blocking and bandpass regions of a variable optical filter differfrom each other in the following manner. In the blocking region, highindex contrast is a key requirement. The high index contrast allows oneto reduce the number of layers and increase the blocking efficiency. Inthe bandpass region, low loss is more important than the high indexcontrast, because light undergoes multiple reflections in that region,traversing the layers of the bandpass region many more times than in theblocking layer. Therefore, providing low-loss, but comparativelylow-index material combinations in the bandpass region, together withhigh index contrast, but comparatively lossy material combinations inthe blocking region, can result in a thin, low-stress variable opticalfilter having simultaneously a low optical loss, a narrowbandtransmission peak, and strong out-of-band rejection.

In accordance with the invention, there is provided an optical filterhaving a laterally variable transmission wavelength within a wavelengthrange, the optical filter comprising:

a bandpass filter comprising a stack of alternating first and secondlayers comprising first and second materials, respectively, and havinglaterally varying thicknesses for providing the laterally variabletransmission wavelength;

a blocking filter comprising a stack of alternating third and fourthlayers comprising third and fourth materials, respectively, and havinglaterally varying thicknesses coordinated with the laterally varyingthicknesses of the first and second dielectric layers, for blockingwavelengths within the wavelength range that are larger or smaller thanthe laterally variable transmission wavelength;

wherein the first, second and fourth materials each comprise differentmaterials, so that a refractive index of the first material is smallerthan a refractive index of the second material, the refractive index ofthe second material is smaller than a refractive index of the fourthmaterial, and an absorption coefficient of the second material issmaller than an absorption coefficient of the fourth material.

Advantageously, the bandpass filter further includes at least one fifthlayer comprising the fourth material and disposed in an area of a localminimum of a standing optical wave inside the bandpass filter, whereby ablocking wavelength region of the bandpass filter is broadened, and athickness of the bandpass filter is reduced.

In one embodiment, the transmission wavelength is monotonically variablealong a length dimension of the optical filter. In a preferredembodiment, the transmission wavelength is logarithmically variablealong the length dimension. The first and third materials can include asame material. The optical filter can include three, four, or moredifferent materials.

In accordance with the invention, there is further provided awavelength-selective sensor comprising the optical filter as describedabove, and an array of photodetectors coupled to the optical filter. Thephotodetectors are spaced apart along the length dimension. As a result,different photodetectors of the array are responsive to differentwavelengths of light impinging on the optical filter from a sideopposite to the array.

Preferably, the array of photodetectors comprises a substrate for theoptical filter. The array of photodetectors is provided, and the firstto fifth layers are deposited directly onto the array, so as to form thebandpass and blocking filters, respectively, supported by the array.Among advantages of depositing the optical filter directly onto thephotodetector array are increased spectral resolution due to light nothaving to propagate through a separate bulk substrate, reduced lightleakage between neighboring photodetectors, and improved reliability.The optical filter can also be manufactured separately and affixeddirectly to the array, e.g. bonded with an optical epoxy.

In accordance with another embodiment of the invention, there is furtherprovided a wavelength-selective sensor, wherein the photodetector arraycomprises a device chip having opposed first and second surfaces. Thephotodetectors of the array are disposed in the first surface of thedevice chip, and the optical filter is disposed on the first surface,over the photodetectors. Such a wavelength-selective sensor may beproduced by

(A) manufacturing the device chip by

(i) providing a device wafer having first and second opposing surfaces;

(ii) forming in the second surface of the device wafer thephotodetectors of the array facing towards the first surface;

(iii) polishing the first surface of the device wafer to expose thephotodetectors of the array; and

(B) depositing on the first surface of the device wafer polished in step(iii) the first to fifth layers, so as to form the bandpass filter andthe blocking filter of the optical filter.

In accordance with another embodiment of the invention, there is furtherprovided a method of manufacturing a wavelength-selective sensor,comprising:

(a) providing an array of photodetectors;

(b) depositing on the array of photodetectors

a bandpass filter comprising a stack of alternating first and secondlayers comprising first and second materials, respectively, and havinglaterally varying thicknesses for providing the laterally variabletransmission wavelength; and

a blocking filter comprising a stack of alternating third and fourthlayers, comprising third and fourth materials, respectively, and havinglaterally varying thicknesses coordinated with the laterally varyingthicknesses of the first and second dielectric layers, for blockingwavelengths within the wavelength range that are larger or smaller thanthe laterally variable transmission wavelength;

wherein the first, second and fourth materials each comprise differentmaterials, so that a refractive index of the first material is smallerthan a refractive index of the second material, the refractive index ofthe second material is smaller than a refractive index of the fourthmaterial, and an absorption coefficient of the second material issmaller than an absorption coefficient of the fourth material.

In one embodiment, step (a) includes:

(i) providing a device wafer having first and second opposing surfaces;

(ii) forming in the second surface of the device wafer the array ofphotodetectors facing the first surface; and

(iii) polishing the first surface of the device wafer to expose thephotodetectors of the array,

wherein in step (b), the bandpass and blocking filters are deposited inthe first surface of the device wafer polished in step (iii).

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which:

FIG. 1A is a three-dimensional view of a prior-art optical spectrometerbased on a thin-film LVF;

FIG. 1B is an enlarged cross-sectional view of the thin film structureof the LVF of FIG. 1A;

FIG. 2 is a cross-sectional view of a variable optical filter of theinvention on a substrate;

FIGS. 3A to 3C are transmission spectra of blocking and bandpass filtersections of the variable optical filter of FIG. 2 at different locationsalong a direction of the transmission wavelength variation;

FIG. 4A is a schematic cross-sectional view of the filter of FIG. 2taking along the lines A-A in FIG. 2, showing the bandpass filter andblocking filter sections;

FIG. 4B is a magnified cross-sectional view of the bandpass section ofthe filter of FIG. 4A;

FIGS. 5A and 5B are transmission spectra of the bandpass filter sectionmade of different materials, at illumination with collimated light (FIG.5A) and at illumination with diverging or converging light (FIG. 5B);

FIG. 6 are transmission spectra of the blocking filter made of differentmaterials;

FIGS. 7A and 7B are spatial refractive index plots of a variable opticalfilter including low-loss oxides only (FIG. 7A) and a combination of thematerials according to the invention (FIG. 7B), drawn to a same scale;

FIG. 7C is an enlarged view of the spatial refractive index plot of FIG.7B;

FIG. 7D is transmission spectra of bandpass sections of the variableoptical filters of FIGS. 7A and 7B;

FIGS. 8A to 8D are side cross-sectional views of various embodiments ofwavelength selective sensors of the invention including the variableoptical filter of FIG. 2 or FIG. 7B;

FIGS. 9A and 9B are side cross-sectional views of embodiments ofwavelength selective sensors having a back-polished photodetector array(FIG. 9A) and a flip-chip mounted multiplexer circuit (FIG. 9B); and

FIGS. 10A to 10C are side cross-sectional views of a wafer includingwavelength selective sensors of FIG. 9A at different stages ofmanufacturing.

DETAILED DESCRIPTION OF THE INVENTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives, modifications and equivalents, as willbe appreciated by those of skill in the art.

Referring to FIG. 2 and FIGS. 3A, 3B, and 3C, an optical filter 200(FIG. 2) has a narrow passband 310 (FIGS. 3A to 3C) at a laterallyvariable transmission wavelength λ_(T) within a wavelength range betweenλ₁ and λ₂. The transmission wavelength λ_(T) is variable in a directionx shown in FIG. 2. The optical filter 200 can be disposed on atransparent substrate 202. The variable optical filter 200 includes abandpass filter 204 and blocking filters 206A, 206B disposed over thesubstrate 202. The bandpass filter 204 comprises a stack of alternatingfirst 211 and second 212 layers including first and second materials,respectively. The first 211 and second 212 layers have thicknessesvarying laterally in the direction x (FIG. 2), for providing thelaterally variable transmission wavelength λ_(T) of the passband 310.The transmission wavelength λ_(T) varies in an approximate proportion tothe local thicknesses of the first 211 and second 212 layers. Forexample, the transmission wavelength λ_(T) is close to the shortestwavelength λ₁ of the wavelength range (λ₁, λ₂) at a coordinate x₁ (FIGS.2 and 3A); is in the middle of the wavelength range (λ₁, λ₂) at acoordinate x₂ (FIGS. 2 and 3B); and is close to the longest wavelengthλ₂ of the wavelength range (λ₁, λ₂) at a coordinate x₃ (FIGS. 2 and 3C).

The blocking filters 206A, 206B each include a stack of alternatingthird 213 and fourth 214 layers including third and fourth materials,respectively. The third 213 and fourth 214 layers have laterally varyingthicknesses coordinated with the laterally varying thicknesses of thefirst 211 and second 212 layers, for blocking wavelengths within thewavelength range (λ₁, λ₂) that are shorter or longer than the laterallyvariable transmission wavelength λ_(T). Specifically, the top blockingfilter 206A is for blocking wavelength shorter than λ_(T) (left-sidebands 301A, 302A, and 303A in FIGS. 3A, 3B, and 3C, respectively), andthe bottom blocking filter 206B is for blocking wavelength longer thanλ_(T) (right-side bands 301B, 302B, and 303B in FIGS. 3A, 3B, and 3C,respectively).

According to the invention, the first, second, and third materials ofthe first 211, second 212 and fourth 214 layers, respectively, allinclude different materials. Typically, the first and second materialsinclude dielectric materials, and the third and fourth materials includedielectric or semiconductor materials. The refractive index of the firstmaterial is smaller than the refractive index of the second material.The refractive index of the second material is smaller than therefractive index of the fourth material. The absorption coefficient ofthe second material is smaller than an absorption coefficient of thefourth material. In other words, the fourth material, although havingthe highest refractive index of all four, can also be somewhatabsorptive. For example, a semiconductor material such a silicon can beused for the fourth material. The first and third materials can, but donot have to, include a same low-index material e.g. silicon dioxide. Thesecond material can include a high-index oxide such as tantalumpentoxide, for example. As a guideline, the refractive index of thefirst and third materials can be between 1.35 and 1.6, the refractiveindex of the second material can be between 1.8 and 2.5, and therefractive index of the fourth material can be between 2.6 and 4.5.

Using different materials for the first 211 and second 212 layers of thebandpass filter 204, and for the fourth layer 214 of the blockingfilters 206A, 206B allows independent optimization of optical parametersof the bandpass filter 204 and the blocking filter 206A, 206B, as willbe explained in detail below. Of course, the blocking filters 206A, 206Bcan also be disposed next to each other, forming a single blockingfilter, with the bandpass filter 204 supported by the single blockingfilter supported by the substrate 202. Furthermore, the thicknesses ofthe layers 211 to 214 shown in FIG. 2 can increase non-linearly, ingoing from left to right in the direction x, to provide a non-linearlylaterally variable transmission wavelength λ_(T) of the optical filter200. In one embodiment, the transmission wavelength λ_(T) islogarithmically variable along the length direction x. The logarithmicvariation of the transmission wavelength λ_(T) results in a constantresolving power along the direction x when a constant-pitchphotodetector array is disposed along the x-direction. The resolvingpower is defined as R=λ_(T)/Δλ, wherein Δλ is a transmission bandwidth.

Referring to FIG. 4A, a sectional view A-A of the variable opticalfilter 200 of FIG. 2 shows the bandpass filter 204, disposed in anoptical path 420 between the first 206A and second 206B blockingfilters, for blocking wavelengths shorter and longer than thetransmission wavelength λ_(T), respectively, of the bandpass filter 204.Each blocking filter 206A and 206B includes three portions 431A, 432A,and 433A; and 431B, 432B, and 433B, respectively. The thickness of thebandpass filter 204 and the blocking filter portions 431A to 433A; and431B to 433B varies in a coordinated fashion, in the direction xperpendicular to the plane of FIG. 4A, as best seen in FIG. 2.

The blocking bands (not shown) of the blocking filter portions 431A to433A; and 431B to 433B are cascaded to cover a broader wavelength range.Typically, it is the blocking wavelength range that dictates the usablewavelength range (λ₁, λ₂) of the variable optical filter 200. At thelong wavelength edge λ₂, the blocking of the portions 431B to 433B ofthe second blocking filter 206B must extend between λ_(T) and λ₂, and atthe short wavelength edge λ₁, the blocking of the portions 431A to 433Aof the first blocking filter 206A must extend between λ₁ and λ_(T). Theout-of-band wavelength blocking afforded by the blocking filters 206Aand 206B is particularly important when the optical frequency range ofthe optical filter 200 spans over one octave, because etalon-typeoptical filters have multiple transmission peaks separated by octaves ofoptical frequency.

Referring to FIG. 4B, the bandpass filter 204 includes a dielectricspacer layer 450 that is a multiple of half-waves at the desired centerwavelength λ_(T), sandwiched between quarter-wave reflector stacks 432at the wavelength λ_(T). The bandwidth of the bandpass filter 204 ismade narrow by increasing the reflectivity of the quarter-wave stacks432 and/or by increasing the thickness, or the number of half-waves, ofthe spacer layer 450. In either case, the transmission bandwidth isreduced by increasing the number of travels of light 440 across thespacer layer 432. Therefore, it is important that the spacer layer 450material, and adjacent layers of the quarter-wave reflector stacks 432,have low optical loss.

Referring back to FIG. 4A with further reference to FIG. 4B, thevariable optical filter 200 (FIG. 4A) is typically used with light thatcontains a range of angles of incidence, or a cone of light. To reducethe effect of shift of the transmission wavelength with an angle ofincidence, it is preferred that the refractive index of the spacer layer450 material (FIG. 4B) be as high as possible. To that end, a high-indexrefractory oxide, such as tantalum pentoxide (Ta₂O₅), niobium pentoxide(Nb₂O₅), an alloy of tantalum pentoxide and niobium pentoxide, ortitanium dioxide (TiO₂), can be used. Metal oxides generally have a verylow optical loss, and tantalum pentoxide, niobium pentoxide, andtitanium dioxide have the refractive index above 2.0 in the wavelengthrange of interest, between 900 nm and 1700 nm. A suitable low-indexmaterial used for the reflector stacks 432 is silicon dioxide (SiO₂),having the refractive index of around 1.5 in the above wavelength range.It is noted that the bandpass filter 204 and/or the blocking filters431A, 431B, 432A, 432B, and 433A, 433B can include different high indexmaterials, and different low index materials. For example, the bandpassfilter 204 can include a combination of silicon dioxide (SiO₂) andtantalum pentoxide (Ta₂O₅), and the blocking filters 431A, 431B, 432A,432B, and 433A, 433B can include a combination of magnesium fluoride(MgF₂) and silicon (Si). Four or more material types can be used,depending upon targeted spectral performance of the optical filter 200.

The degree of light blocking and bandwidth of the blocking filterportions 431A to 433A and 431B to 433B is set by so called indexcontrast, or a ratio of the refractive indices of high and low-indexlayers of the blocking filter portions 431A to 433A and 431B to 433B. Byincreasing the index contrast, the overall thickness of the blockingfilter portions 431A to 433A and 431B to 433B can be reduceddramatically, both because fewer layers are needed to achieve a desiredblocking level, and because the blocking filter portions 431A to 433Aand 431B to 433B will have a wider bandwidth, so that fewer stacks areneeded to cover a desired bandwidth. The light 440 (FIG. 4B) propagatingalong the optical path 420 (FIG. 4A) does not travel across the blockingfilter portions 431A to 433A and 431B to 433B as many times as in thebandpass filter 204, so a wider range of materials can be used, andspecifically, silicon (Si) is preferably used. Silicon has therefractive index of over 3.0, and can be used as the high indexmaterial, even though is has some optical absorption in the wavelengthrange between 900 nm and 1700 nm. It is also naturally compatible withsilicon dioxide, which has low refractive index, thus providing thesought-for high index contrast in the blocking filter portions 431A to433A and/or 431B to 433B.

The above conclusions about wavelength selectivity and optical loss ofdifferent material systems have been confirmed experimentally, bygrowing multilayer stacks and measuring their transmission properties.Referring to FIGS. 5A and 5B, optical transmission spectra in collimatedlight (FIG. 5A) and non-collimated or “cone” light (FIG. 5B) of thebandpass filter 204 manufactured using different material combinations,are presented. In FIGS. 5A and 5B, “H/L” denotes high-index/low-indexmaterial combination of tantalum pentoxide and silicon dioxide, and“S/L” denotes the material combination of silicon and silicon dioxide.Accordingly, “L cav” denotes the “cavity”, or the spacer layer 450 madeof the low-index material, that is, silicon dioxide. “H cav” denotes thespacer layer 450 made of tantalum pentoxide. “S cav” denotes the spacerlayer 450 made of silicon.

Referring specifically to FIG. 5A, spectra 501 and 502 correspond to thematerial combination of tantalum pentoxide and silicon dioxide with thesilicon dioxide and tantalum pentoxide spacer layer 450, respectively.One can see that in collimated light, the spectra 501 and 502 arepractically indistinguishable from each other and show a very high(approaching 100%) maximum transmission. Spectra 503 and 504 correspondto the material combination of silicon and silicon dioxide with thesilicon dioxide and silicon spacer layer, respectively. The spectrum503, corresponding to the silicon dioxide spacer layer 450, shows ahigher maximum transmission (about 67%), than the spectrum 504corresponding to the silicon spacer layer 450 (about 38%). This isbecause, as noted above, silicon has much higher optical absorption thaneither silicon dioxide or tantalum pentoxide and, since the light 440(FIG. 4B) traverses the spacer layer 450 many times, the difference inthe optical transmission becomes very noticeable (in this example 38% or67% vs. 100%).

Turning now to FIG. 5B, all spectra 501 to 504 show increased opticalloss due to the cone light illumination. Out of the first two spectra501 and 502, the second spectrum 502, corresponding to the tantalumpentoxide spacer layer 450, shows a lower transmission drop of about 32%(from 100% to about 68% maximum transmission) than the first spectrum501 corresponding to the silicon dioxide spacer layer 450, showing a 41%transmission drop (from 100% to about 59% maximum transmission). Asexplained above, the higher the refractive index of the spacer layer 450material, the smaller the angular sensitivity of the bandpass opticalfilter 204. A similar trend is observed in the other two optical spectra503 and 504, corresponding to silicon/silicon dioxide materialcombinations. For the spectrum 504, corresponding to the silicon spacerlayer 450, the optical transmission drops by only about 3% (from 38% to35%), while for the spectrum 503, corresponding to the silicon dioxidespacer layer 450, the optical transmission drops by about 17% (from 67%to 50% transmission). Thus, low-loss, but high refractive index materialshould be selected for the spacer layer 450 of the bandpass opticalfilter 204.

Referring now to FIG. 6 with further reference to FIG. 4A, transmissionspectra 601 and 602 of the lower blocking filter 206B made out oftantalum pentoxide/silicon dioxide and silicon/silicon dioxide materialcombinations, respectively, are presented. A bandwidth 611 of the firstspectrum 601 is only 275 nm, which is much narrower than a bandwidth 612of the second spectrum 602, which is 664 nm. Thus, high index contrastmaterial combinations should be selected for the blocking filters 206Aand 206B. It is noted that the wider bandwidth of the second spectrum602 is achieved at a much thinner filter, only 1.7 micrometers forsilicon/silicon dioxide stack, as compared to 4.9 micrometers thicknessof tantalum pentoxide/silicon dioxide stack.

The variable filter thickness reduction afforded by the three-materialsystem (in going from the lowest refractive index to the highest:silicon dioxide, tantalum pentoxide, and silicon) will now beillustrated. Turning to FIGS. 7A and 7B, spatial refractive index plots700A and 700B are dependencies of refractive index n as a function of astack depth coordinate d. In FIGS. 7A and 7B, the spatial refractiveindex plots 700A and 700B are drawn to a same scale to illustrate thetotal thickness reduction achievable by this invention. The refractiveindex plot 700A of FIG. 7A corresponds to a variable optical filterimplemented using silicon dioxide/tantalum pentoxide two-materialsystem. Silicon dioxide layers are represented by lower black bars 701,and tantalum pentoxide layers are represented by higher gray bars 702.The refractive index plot 700A includes a first blocking section 726A, abandpass section 724, and a second blocking section 726B. The bandpasssection 724 includes two thicker tantalum pentoxide layers 702 thatfunction as bandpass filter cavity (spacer) layers 702A. The bandpasssection 724 is, therefore, a two-cavity bandpass filter. Each spacerlayer 702A has a reflector stack on each side; the reflector stacks aresomewhat symmetrical about the spacers. The reflector stacks in betweenthe two spacers 702A combine into one reflector stack 724C. The totallength of the refractive index plot 700A of FIG. 7A, corresponding tothe thickness of a two-material variable optical filter, is as large as40 micrometers.

The refractive index plot 700B of FIG. 7B corresponds to a variableoptical filter implemented using silicon dioxide/tantalumpentoxide/silicon three-material system. Referring to FIG. 7C, which isa magnified view of FIG. 7B, the silicon dioxide layers are representedby the lowest black bars 701, the tantalum pentoxide layers arerepresented by the higher gray bars 702, and the silicon layers arerepresented by the highest black bars 703.

The refractive index plot 700B includes a first blocking section 746A, abandpass section 744, and a second blocking section 746B. Referringmomentarily back to FIG. 2, the first blocking section 746A correspondsto the first blocking filter 206A, the bandpass section 744 correspondsto the bandpass filter 204, and a second blocking section 746Bcorresponds to the second blocking filter 206B of the optical variablefilter 200 of FIG. 2. The first 746A and second 746B blocking sectionsinclude alternating silicon dioxide layers 701 and the silicon layers703. The bandpass section 744 includes the silicon dioxide layers 701,the tantalum pentoxide layers 702 and the silicon layers 703. Onedifference between the bandpass section 744 of the optical filter 700Bof FIGS. 7B and 7C and the bandpass filter 204 of the optical filter 200of FIG. 2 is that the bandpass section 744 of the optical filter 700B isa two-cavity bandpass filter, similarly to the two-cavity bandpasssection 724 of the optical filter 700A of FIG. 7A using, however, nottwo but three different materials, as explained below.

The bandpass section 744 includes first 744A and second 744B cavities,each including the tantalum pentoxide spacer 702A between twoquarter-wave reflector sections. A silicon layer 751 is introduced intoeach reflector section. As the reflector sections are somewhatsymmetrical about the spacer layer 702A, this adds four silicon layers751 to the bandpass section 744, as shown. Introducing at least one, andpreferably several optional high-index silicon layers in place of the Hlayers 702 into the quarter-wave reflector sections of the bandpasssection 744 allows the same reflectance to be achieved with fewerlayers, due to the higher index ratio of silicon to silicon dioxide(S/L) compared to tantalum pentoxide to silicon dioxide (H/L). Theoptical transmission loss due to inclusion of the four additionalsilicon layers 751 can be reduced by placing the additional siliconlayers 751 in area(s) corresponding to local minima of optical field,that is, in the valley of the standing optical wave at the transmissionwavelength λ_(T) inside the bandpass section 744. The total length ofthe refractive index plot 700B of FIG. 7B, corresponding to thethickness of a three-material variant of the variable optical filter 200of FIG. 2, is only 10 micrometers, that is, four times thinner than inFIG. 7A.

One further advantage of the additional silicon layers 751 is that thehigher index ratio of the silicon/silicon dioxide combination broadens ablocking region of the bandpass filter section 744 and reduces requiredblocking wavelength bands of the lower 746A and upper 746B blockersections. Referring to FIG. 7D, the transmission scale is 0% to 1%transmission, to better show the stopband performance. A dashed line 781is a transmission plot of the bandpass filter section 724 of thevariable optical filter 700A of FIG. 7A implemented with two materials(tantalum pentoxide and silicon dioxide). A solid line 782 is atransmission plot of the bandpass filter section 744 implemented asshown in FIG. 7C. One can see that using the optional silicon layers 751allows one to considerably expand the blocking bandwidth, and theblocking strength, of wings 782A, 782B of the bandpass spectrum 782. Allthis achieved at a smaller overall thickness of the bandpass filtersection 744 of the variable optical filter 700B of FIGS. 7B and 7C.

Referring back to FIG. 2, the resulting thickness of the filter 200depends on the materials used, and on the target optical specifications.For example, the first layer 211 of the bandpass optical filter 204 caninclude silicon dioxide, the second layer 212 can include tantalumpentoxide or niobium pentoxide, the third layer of the blocking opticalfilter 206 can also include silicon dioxide (same as the first layer211), and the fourth layer 214 of the blocking optical filter 206 caninclude silicon. The resulting three-material system allows one toreduce the thickness of the variable optical filter 200. For anear-infrared wavelength range of 900 nm to 1700 nm, the bandpass filter204 can include no more than 20 layers, and the blocking filters 206A,206B can include the total of no more than 60 layers. The totalthickness of the filter 200 is preferably no greater than 20 micrometersat a location corresponding to the transmission wavelength of 1300 nm,and more preferably no greater than 10 micrometers. The number of layersand thickness will be driven by many factors, such as bandpass width,blocking level required, and the wavelength coverage of the variableoptical filter 700B.

Turning now to FIG. 8A with further reference to FIG. 2, awavelength-selective sensor 800A of the invention includes the variableoptical filter 200 of FIG. 2 or 700B of FIGS. 7B, 7C, and aphotodetector array 802A coupled to the variable optical filter 200 withphotodetectors 812 spaced apart along the x direction, in which thelayer thickness monotonically increases. Since the transmissionwavelength λ_(T) varies along the direction x, different photodetectors812 of the photodetector array 802A are responsive to differentwavelengths of light 840 impinging on the optical filter 200 from a side820 opposite to the photodetector array 802A. The number of thephotodetectors 812 can vary from only two or three photodetectors, fordetecting several isolated wavelength bands, to hundreds or morephotodetectors, for performing detailed optical spectral measurements.In the latter case, the wavelength-selective sensor 800A essentiallyfunctions as an optical emission spectrometer.

In the embodiment of a spectrometer, when the number of photodetectors812 is in tens, hundreds, or higher number of photodetectors, thetransmission wavelength λ_(T) can be made logarithmically variable alongthe length dimension x of the variable optical filter 200. When thelogarithmic variation the transmission wavelength λ_(T) is combined witheven spacing of photodetectors 812 of the photodetector array 802A, theresolving power R=λ_(T)/Δλ of spectral points collected by the opticalspectrometer 800 is a constant value, which may be preferable e.g. forspace applications.

The photodetector array 802 can include a substrate for the opticalfilter 200. In other words, the photodetector array 802 can function asa substrate supporting the optical filter 200. A gap 814 between theoptical filter 200 and the photodetector array 802 can be filled with anoptional adhesive layer. Alternatively, a mechanical encasing, notshown, can be used to support the optical filter 200 over thephotodetector array 802. In the latter embodiment, the gap 814 caninclude vacuum, air, gas, etc. Furthermore, the photodetector array 802can be used a substrate during deposition of the first 211 to fourth 214layers of the optical filter 200. In this embodiment, the gap 814 can,but does not have to, include a planarization layer, for a betteruniformity of deposition of the first 211 to fourth 214 layers. When theplanarization layer fills the gap 814, different photodetectors 812 ofthe array 802 can have different heights. For instance, referring toFIG. 8B, photodetectors 812A, 812B, and 812C of an array 802B of awavelength-selective sensor 800B have different heights, and theplanarization layer filling the gap 814 ensures that the optical filter200 is deposited onto an even surface 815.

Referring to FIG. 8C, a wavelength-selective sensor 800C of theinvention is similar to the wavelength-selective sensors 800A and 800Bof FIGS. 8A and 8B, respectively. In the wavelength-selective sensor800C of FIG. 8C, the photodetectors 812 of a photodetector array 802Care separated by laterally spaced gaps 813, and an opaque isolationmaterial 817 is disposed in the gaps 813, for electrical and/or opticalisolation of the individual photodetectors 812 of the array 800C. Theopaque isolation material can include a black or electrically-isolatingepoxy, e.g. 353NDB epoxy manufactured by Epoxy Technology,Massachusetts, USA.

Turning to FIG. 8D, a wavelength-selective sensor 800D of the inventionis similar to the wavelength-selective sensors 800C of FIG. 8C. In thewavelength-selective sensor 800D of FIG. 8D, portions of the opticalfilter 200 are removed, e.g. etched through, forming slots 816 eachdisposed directly above a corresponding one of the gaps 813 between thepixels 812 of a photodetector array 802D, and the optional opaqueisolation material 817 is disposed in the slots 816, for better opticalisolation and reduction of optical crosstalk between the individualphotodetectors 812.

Referring to FIG. 9A, a wavelength-selective sensor 900A of theinvention is a variant of the wavelength-selective sensor 800A of FIG.8A. The wavelength-selective sensor 900A of FIG. 9A includes the opticalfilter 200 of FIG. 2 and an array 902A of photodetectors 912A. The array902A is effectively a substrate for the optical filter 200. The opticalfilter 200 can be disposed directly on the array 902A, thus obviating aneed of a separate thick substrate, e.g. a separate glass substrate atleast 0.8 mm thick, for supporting the optical filter 200. The array902A includes a device chip 920 having opposed first 921 and second 922surfaces. The photodetectors 912A of the array 902A are disposed in thefirst surface 921 of the device chip 920. The optical filter 200 isdisposed on the first surface 921 over the photodetectors 912A. Bondpads 929A can extend through the device chip 920 e.g. on opposite sidesof the optical filter 200, as shown in FIG. 9A. A carrier chip 930 canbe bonded to the second surface 922 of the device chip 920, forstrengthening the device chip 920 during optional polishing of thedevice chip 920, see below.

Turning now to FIG. 9B, a wavelength-selective sensor 900B of theinvention is a variant of the wavelength-selective sensors 800A of FIG.8A and 900A of FIG. 9A. The wavelength-selective sensor 900B of FIG. 9Bincludes the optical filter 200 of FIG. 2 and an array 902B ofphotodetectors 912B. The array 902B includes an array substrate 940having opposed first 941 and second 942 surfaces. The photodetectors 912of the array 902B are disposed within the first surface 941, and theoptical filter 200 is disposed on the second surface 942 of the arraysubstrate 940. In this embodiment, the light 840 reaches thephotodetectors 912B of the array 902B by propagating through the arraysubstrate 940.

In a preferred embodiment, the wavelength-selective sensor 900B alsoincludes a multiplexer chip 950 including multiplexer circuitry 955 forreading photoelectric signals of the photodetectors 912B of the array902B. The multiplexer chip 950 is flip-chip bonded to the first surface941 of the array substrate 940. Bond pads 929B can extend between themultiplexer chip 950 and the array substrate 940, for establishingelectrical contacts between the multiplexer chip 950 and thephotodetectors 912B of the array 902B.

The variable optical filter 700B of FIG. 7B can be used instead of thevariable optical filter 200 in the wavelength-selective sensors 800A to800D, 900A, 900B of FIGS. 8A to 8D and 9A, 9B respectively. The filters200, 700B can be implemented not only in a three-material system, butalso in a material system including four and more materials. The firstmaterial can include silicon dioxide, the second material can includetantalum pentoxide, and the third material can include silicon. With aproper selection of materials, the variable optical filter 200 can havea thickness of no greater than 20 micrometers, and preferably 10micrometers or less, greatly reducing mechanical stresses in thevariable optical filter 200 and increasing manufacturing yields.

Preferably, the variable optical filters 200 or 700B are disposeddirectly on the photodetector arrays 802A to 802D; 902A, and 902B ofFIGS. 8A to 8D; 9A, and 9B, respectively. In these embodiments, thephotodetector arrays 802A to 802D; 902A, and 902B are effectivelysubstrates for the variable optical filters 200 or 700B duringdeposition of the first 211 and second 212 layers in alternation and thethird 213 and fourth 214 layers in alternation, so as to form thebandpass 204 and blocking filters 206A, 206B, respectively, on theprovided photodetector arrays 802A to 802D; 902A, and 902B. Disposingthe optical filters 200 or 700B directly on the photodetector arrays802A to 802D; 902A, and 902B of FIGS. 8A to 8D; 9A, and 9B results in amore compact overall construction and improves spectral resolution ofthe corresponding wavelength-selective sensors 800A to 800D, 900A, and900B, since light travels a shorter distance to the photodetectors 812,812A, 812B, 812C, 912A, and 912B, and diverges less as compared to theembodiment of the optical filter 200 including the optional bulksubstrate 202 (FIG. 2).

Referring to FIGS. 10A, 10B, and 10C with further reference to FIG. 9A,a method of manufacturing of the wavelength-selective sensor 900A (FIG.9A) is illustrated. A device wafer 1020 (FIG. 10A) is provided havingfirst 1021 and second 1022 opposing surfaces. Then, the photodetectors912A and the optional bond pads 929A are formed in the second surface1022 such that the photodetectors 912A face the first surface 1021 frominside of the device wafer 1020 (FIG. 10A). Then, the device wafer 1020is bonded to an optional carrier wafer 1030, and the first surface 1021is polished off so as to expose the photodetectors 912A of the array902, as well as the bond pads 929A (FIG. 10B). The carrier wafer 1030 isbonded to provide mechanical strength for ease of polishing. Then, theoptical filter 200 is deposited on the first surface 1021 (FIG. 10C)between the bond pads 929A, layer by layer. The device wafer 1020 canthen be diced into individual device chips 920.

The foregoing description of one or more embodiments of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

What is claimed is:
 1. An optical filter comprising: a transparentsubstrate, a first filter comprising a stack of alternating first layersand second layers, the first layers including a first material, thesecond layers including a second material, and the stack includes atleast one of silicon, silicon dioxide (SiO₂), or tantalum pentoxide(Ta₂O₅), and a second filter, the second filter including a third layerand a fourth layer, and the first layers, the second layers, and thefourth layer including different materials.
 2. The optical filter ofclaim 1, wherein the third layer and the fourth layer have laterallyvarying thicknesses.
 3. The optical filter of claim 1, wherein the firstlayers and the second layers have thicknesses varying laterally in anx-direction.
 4. The optical filter of claim 1, wherein a refractiveindex of the first material is smaller than a refractive index of thesecond material.
 5. The optical filter of claim 1, wherein a refractiveindex of the first material is between 1.35 and 1.6.
 6. The opticalfilter of claim 1, wherein the second filter includes a third material,and wherein a refractive index of the third material is between 1.35 and1.6.
 7. The optical filter of claim 1, wherein a refractive index of thesecond material is between 1.8 and 2.5.
 8. The optical filter of claim1, wherein the second filter includes no more than 60 layers.
 9. Anoptical filter comprising: a first filter comprising a stack ofalternating first layers and second layers, the first layers including afirst material, the second layers including a second material, and thestack includes at least one of silicon, silicon dioxide (SiO₂), ortantalum pentoxide (Ta₂O₅), and a second filter, wherein the secondfilter includes a third material, and wherein a refractive index of thethird material is between 2.6 and 4.5.
 10. A variable optical filtercomprising: a transparent substrate, a bandpass filter comprising astack of alternating first layers and second layers, the first layersincluding a first material, and the second layers including a secondmaterial, and one or more blocking filters, the one or more blockingfilters including a first blocking filter and a second blocking filter.11. The variable optical filter of claim 10, wherein the first layersand the second layers have thicknesses varying laterally in anx-direction in a manner that provides a laterally variable transmissionwavelength of a passband.
 12. The variable optical filter of claim 11,wherein the laterally variable transmission wavelength varies in anapproximate proportion to a local thickness of the first layers and thesecond layers.
 13. An optical filter comprising: a bandpass filtercomprising a stack of alternating first layers and second layers, thefirst layers including a first material, and the second layers includinga second material, and one or more blocking filters, a blocking filter,of the one or more blocking filters, including a stack of alternatingthird layers and fourth layers.
 14. The optical filter of claim 13,wherein the third layers include a third material, and wherein thefourth layers include a fourth material.
 15. The optical filter of claim14, wherein a first refractive index of the first material and the thirdmaterial is between 1.35 and 1.6, and wherein a second refractive indexof the fourth material is between 2.6 and 4.5.
 16. The optical filter ofclaim 13, wherein the third layers and the fourth layers have laterallyvarying thicknesses coordinated with laterally varying thicknesses ofthe first layers and the second layers.
 17. An optical filtercomprising: a bandpass filter comprising a stack of alternating firstlayers and second layers, the first layers including a first material,and the second layers including a second material; and one or moreblocking filters, the bandpass filter including a combination of silicondioxide (SiO₂) and tantalum pentoxide (Ta₂O₅).
 18. An optical filtercomprising: a bandpass filter comprising a stack of alternating firstlayers and second layers, the first layers including a first material,and the second layers including a second material; and one or moreblocking filters, the one or more blocking filters including acombination of magnesium fluoride (MgF₂) and silicon (Si).
 19. Theoptical filter of claim 18, wherein the bandpass filter includes acombination of silicon dioxide (SiO₂) and tantalum pentoxide (Ta₂O₅).20. The optical filter of claim 18, wherein the first layers and thesecond layers have thicknesses varying laterally in an x-direction.